Misinitiation of intrathymic MART‐1 transcription and biased TCR usage explain the high frequency of MART‐1‐specific T cells

Abstract

Immunity to tumor differentiation antigens, such as melanoma antigen recognized by T cells 1 (MART-1Additional supporting information may be found in the online version of this article at the publisher's web-site IntroductionThe immune response against melanoma-associated tumor antigens has been intensely studied in part owing to the high Eur. J. Immunol. 2014Immunol. . 44: 2811Immunol. -2821 antigens in the case of infection-induced cancer; and ectopically or over-expressed cell lineage-specific antigens. One example of the latter category is melanoma antigen recognized by T cells 1 (Melan-A/MART-1, referred to in the following text as MART-1) [1,2], which is expressed by cells of the melanocytic lineage and the majority of melanomas. MART-1 is unusual as a tumor antigen in that CD8 + T cells specific to the peptide-MHC class I complex and MART-1 26-35 -HLA-A2, are about 100 times more frequent in healthy individuals than T cells specific to other self-antigens [3]. This high frequency could be due either to an increased thymic output or to peripheral T cell expansion. As MART-1-specific CD8 + T cells seem to cross-react with environmental antigens, it was speculated that they expand after bacterial or viral infection [4]. T cells owing to the biased usage of the TRAV12-2 variable gene of the TCRα-chain that imparts broad cross-reactivity to other selfantigens including those presented by cortical thymic epithelial cells (cTECs) [6]. However, experimental models have shown that the process of negative selection is exquisitely efficient and even a postselection repertoire consisting of more than 10% auto-reactive TCRs can be efficiently purged by rare antigen-positive medullary antigen presenting cells (APCs) [7,8]. If this were also the case for the human thymus, it would argue against the notion that overt positive selection of MART-1 26-35 -specific CD8 + T cells alone could explain their unusually high frequency. Although quantitative aspects of positive selection cannot be directly addressed experimentally in humans, the prerequisites for tolerance induction, i.e. expression and presentation of MART-1 by thymic APCs, are amenable to ex vivo analysis. In this context, a recent study reported on the linkage between thymic expression of two murine melanocyte-specific tissue restricted antigens (gp100 and TRP-2) and self-tolerance versus antimelanoma reactivity [9]. Initially it had been reported that MART-1 was not expressed in the human thymus [2], yet subsequent studies on promiscuous gene expression (pGE) revealed MART-1 transcripts in human medullary thymic epithelial cells (mTECs) [10,11]. However, these studies left open whether the 5 -located MART-1 26-35 epitope was actually transcribed in mTECs. PGE differs from tissue-specific gene regulation in many aspects, as reflected in different splicing patterns, alternative promoter usage, misinitiated transcription, and its independence of tissue-specific transcription factors [12][13][14]. These "aberrations" can result in truncated transcripts, which may...

Role of the MHC restriction during maturation of antigen‐specific human T cells in the thymus

“…The p values were calculated using two-tailed Mann-Whitney tests (n = 9 A2 + ; n = 8 A2 − ). p<0,0001 for MelA 27 and PGT 178 SP8, p = 0.0003 for NS3 1073 SP8, p = 0.0037 for PGT 178 SP4.…”

“…In this regard, MelA and PGT peptides, which are both derived from self-proteins, are recognized at much higher frequencies than the other immunodominant peptides herein studied, which were all derived from foreign organisms. Along this line, thymic expression of mRNA encoding for MelanA has been previously reported [25,26], although this mRNA might be in most cases truncated, precluding expression of the MelA peptide [27]. The preferential selection of the corresponding Ag-specific T-cell subsets in A2 + donors might thus result from TCR engagement by relevant or closely related p-A2 complexes expressed at densities low enough in the thymus to avoid T-cell deletion [28,29].…”

Reduced naïve CD 8 + T ‐cell priming efficacy in elderly adults

“…To circumvent this biological obstacle to the reliable study of antigen‐specific priming in vitro , we exploited the observation that individuals expressing the HLA‐A2 allotype harbor exceptionally high frequencies of precursors (i.e., 10–100 naïve cells per million CD8 + T‐cells) specific for the heteroclitic Melan‐A/MART‐1 epitope ELAGIGILTV (ELA from hereon) (Dutoit et al ., 2002; Zippelius et al ., 2002). This large antigen‐specific pool in the naïve CD8 + T‐cell compartment provides a unique means to study the priming of human T‐cells that recognize ELA, used here as a model antigen (Pinto et al ., 2014; Romero et al ., 2014). It is also notable that the Melan‐A/MART‐1 epitope is an important melanoma‐associated antigen.…”

Twelve-year survival and immune correlates in dendritic cell–vaccinated melanoma patients

“…It also presents responses of cohort 1 (i.e., ± CD40L pulsing of class I or II peptide-loaded DCs) or cohort 2 (i.e., ± KLH loading of class I peptideloaded DCs) ( Figure 2B). Data ( Figure 2A) clearly reveal that preexisting tumor-specific responses are low or absent at the beginning, except those for the HLA-A2/Melan-A epitope, for which precursors are known to be frequent even in healthy individuals (31,32). Upon vaccination, responses substantially increase but tend to be slightly lower in the maintenance phase of the trial, likely due to the increasing vaccine intervals (Figure 1).…”